The present invention relates to an imaging system comprising an improved electrophotographic imaging member which exhibits enhanced photo-electrical sensitivity upon exposure to activating radiation.
Typical electrophotographic imaging members include photosensitive members or photoreceptors commonly utilized in electrophotographic or xerographic processes in either a flexible belt or rigid drum configuration. The flexible belt may be seamless or seamed.
These electrophotographic imaging members comprise a photoconductive layer comprising a single layer or composite layers. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer capable of photogenerating holes and injecting photogenerated holes sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. Obviously, the supporting electrode may still function as an anode when the charge transport layer is sandwiched between the supporting electrode and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this latter embodiment must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
However, in a positively charged photosensitive member, the supporting electrode may also function as a cathode and the charge transport layer is sandwiched between the contiguous photoconductive layer and the supporting electrode. The photoconductive layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer which is then capable of supporting and transporting the injected holes across the entire thickness of the charge transport layer to reach the supporting electrode.
Various combinations of materials from charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more diamine compounds. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer are well known in the art. The charge generation layer may be homogeneous photoconductive material or a dispersion of photoconductive particles dispersed in a film forming binder as disclosed, for example, in U.S. Pat. No. 4,265,990, the disclosure thereof being incorporated herein in its entirety. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One typical type of multilayered imaging member that has been employed as a belt in electrophotographic imaging systems is a photoreceptor comprising a supporting substrate, a conductive layer, a hole blocking layer, an adhesive layer, a charge generating layer, a charge transport layer, and a conductive ground strip layer adjacent to one edge of the imaging layers. This imaging member may also comprise additional layers, such as an anti-curl back coating layer to the back of the supporting substrate opposite to the side of the active electrophotographic layers, to render the desirable imaging member flatness. An optional overcoating layer may also be used to protect the exposed charge transport layer from wear.
During machine operation, a photoconductive imaging member is constantly subjected to repetitive electrophotographic cycling conditions which subject the electrically operative layers to extensive electrical charging/discharging cycles, multiple exposures to light for latent imaging development and erasure, and heat due to temperature elevation resulting from machine operation. These repetitive electrical and light cycles fatigue the imaging member and lead to a gradual deterioration in the electrical characteristics of the imaging member and limit its service life in the field. In the attempt to fabricate a robust photoconductive imaging system, many innovative ideas have been attempted to overcome these shortfalls and to extend the electrical functional life of the imaging member.
One of the more encouraging advances in electrophotographic imaging member development that has emerged in recent years is the successful fabrication of a novel photoreceptor design which exhibits a nearly ideal capacitive charging characteristic, good photosensitivity, low electrical potential dark decay, and long term electrical cyclic stability. This novel photoreceptor design employed in belt form comprises a substrate, a conductive layer, a solution coated hole blocking layer, a solution coated adhesive layer, a thin vacuum sublimation deposited charge generation layer comprising pure organic pigment of benzimidazole perylene, a solution coated charge transport layer with an adjacent solution co-ground strip at one edge of the imaging layers, a solution anti-curl layer, and an optional overcoating layer.
Although this novel multi-layered photoreceptor device provides excellent electrical properties and extended life, it exhibits a major charge generation layer mud-cracking problem. The observed charge generating layer mud-cracks consists of a two-dimensional network of cracks. Mud-cracking is believed to be the result of built-in internal stress due to the vacuum sublimation deposition process and solvent penetration through the thin charge generation layer which dissolves the underlying adhesive layer during application of the charge transport layer solution. Cracking in the charge generation layer has a serious impact on the versatility of a photoreceptor and reduces its practical value. Charge generation layer mud-cracks not only can print out as defects, but may also act as stress concentration centers which propagate the cracks into the other electrically operative layer, i.e. the charge transport layer, during dynamic imaging member belt machine cycling.
While the above-mentioned imaging member gives the desirable electrical characteristic, there is an urgent need to resolve the cracking issue in order to render the imaging member design acceptable for long term cycling systems. Subsequently formulated innovative concepts directed to elimination of the problem have resulted in varying degrees of success. The most promising approach involves employing a solution dispersion coating technique, by means of which benzimidazole perylene particles are dispersed in a solution consisting of a binder polymer dissolved in a common organic solvent and the pigment dispersion such that after solution coating and drying at elevated temperatures, a crack-free binder generation layer comprising homogeneously dispersed benzimidazole perylene pigment in a polymeric matrix is obtained. The resulting photoreceptor device retains most of its excellent photo-electrical characteristics, but exhibits the major shortfall of having lower photosensitivity than a photoreceptor counterpart fabricated using sublimation deposited benzimidazole perylene charge generation layer. Since low photosensitivity photoreceptor requires a higher power exposure light source to carry out the xerographic imaging process, it increases the manufacturing cost of the machine, exacerbates the reflection interference fringes problem, and causes a reduction in image resolution which substantially and adversely affect the quality of print-out copies.